A port is a transport address for some transport protocols (TCP and UDP), much like addresses for network or data-link addresses. Also, understand that TCP and UDP use the same port number range, but they are not the same ports; TCP port 12345 is not UDP port 12345.
An application will request of TCP or UDP to get a connection with the transport protocol, and either get a random available port number, or a port number specifically requested by the application. Any data sent to that transport protocol at that port number will be sent to the application process that requested that association.
UDP is a connectionless protocol, and it will simply send or receive data for the associated application process with no guarantees of delivery. TCP, on the other hand, is a connection-oriented protocol, and it establishes connections, providing certain guarantees of in-order data delivery. An application process using a TCP port can establish multiple connections on the TCP port. This is explained in RFC 793, Transmission Control Protocol:
Multiplexing:
To allow for many processes within a single Host to use TCP
communication facilities simultaneously, the TCP provides a set of
addresses or ports within each host. Concatenated with the network and
host addresses from the internet communication layer, this forms a
socket. A pair of sockets uniquely identifies each connection. That
is, a socket may be simultaneously used in multiple connections.
The binding of ports to processes is handled independently by each
Host. However, it proves useful to attach frequently used processes
(e.g., a "logger" or timesharing service) to fixed sockets which are
made known to the public. These services can then be accessed through
the known addresses. Establishing and learning the port addresses of
other processes may involve more dynamic mechanisms.
Connections:
The reliability and flow control mechanisms described above require
that TCPs initialize and maintain certain status information for each
data stream. The combination of this information, including sockets,
sequence numbers, and window sizes, is called a connection. Each
connection is uniquely specified by a pair of sockets identifying its
two sides.
When two processes wish to communicate, their TCP's must first
establish a connection (initialize the status information on each
side). When their communication is complete, the connection is
terminated or closed to free the resources for other uses.
Since connections must be established between unreliable hosts and
over the unreliable internet communication system, a handshake
mechanism with clock-based sequence numbers is used to avoid erroneous
initialization of connections.
This allows something like a web server on TCP port 80 to create multiple connections with multiple web browsers because each connection is uniquely identified by the source IP and TCP addresses.
- does occupying all the TCP/UDP ports of a system/node with active
connections indicating that no other upcoming network connection
behavior can be established?
No. UDP is connectionless, and TCP can have many connections on a single port.
- regarding to the previous question, if so, can I consider this
system/node safe from malicious(hacking) network connections that
are knocking the door?
No, not at all.
- can the services or applications within a system/node rob the ports
that are currently in use? for example, the port 54321 is in use by
a service but another program take control of it. Is there any
standards to restrict such behavior?
No, a port assigned to an application process is unusable by any other process until the owning process releases it.
- how does the port 80 operate for different browsers in the client
that are connecting a same http webpage at the same time?does the
web server still provide connections through port 80? or maybe I
misunderstood something?
That is explained above in the RFC. Each browser will have a random address and its own IP address so that the connection with that browswer is distinguished from other connections by other browsers.
